The neuroendocrine bag cells of Aplysia and the reproductive organ called the atrial gland produce a family of closely related peptides which are involved in the initiation and regulation of egg-laying behavior in this animal. Some of the peptides from both sources cause egg release from the gonad and also have directed effects on identified case cells: bag cell egg-laying hormone (ELH) acts directly on the gonad and also excites cell R15; Alpha-bag cell peptide (Alpha-BCP) causes inhibition of the left upper quadrant (LUQ) cells and also excites the bag cells. Atrial gland peptides A and B, portions of which are homologous to Alpha-BCP, also excite the bag cells. The atrial gland peptide, egg-releasing hormone (ERH), ehich is partially homologus to both ELH and peptides A and B, can both act on the gonad and cause excitation of the bag cells. The atrial gland releases several other biologically active peptides, causing either egg laying, bag cell firing, or both, but their ability to affect other neurons has not been tested. We have shown that Alpha-BCP inhibits cells R2, R3-R13, and L10 (interneuron I), and that atrial gland extracts excite the LUQ cells and R2, effects opposite to those caused by the bag cells. Using synthetic peptides or peptide fragments of closely homologous molecules, such as Alpha-BCP and the 9-amino acid C-terminal portion of peptide B (B26-34), we have found that differences in only a few amino acid residues can result in a selective loss of effects: while both Alpha-BCP and peptide B can excite the bag cells, B26-34 only weakly sitmulates Alpha-BCP's inhibitory effect on the LUQ cells. We propose to extend these studies using intracellular electrophysiological and pharmacological techniques, measuring voltage, current and conductance changes in a wide variety of Aplysia neurons, to determine the mechanism of action, ionic dependence, the active site, and the structure-activity relations of several natural and synthetic peptides (or fragments/analogues of them), including ELH, peptide B, ERH and Alpha-BCP. Peptides will be administered by either bath superfusion, arterial perfusion, or localized pressure ejection from microelectrodes. These studies will provide new information about the neural effects of many of the bag cell and atrial gland peptides, and also reveal how alterations in primary structure of neuroactive peptides can influence their mode of action.